1. Industrial Useful Field
This invention relates to a sealed lead acid battery.
2. Prior Art
Since a sealed lead acid battery is so constructed that oxygen gas produced on the positive plate at the time of charging is absorbed in the negative plate, there is generally no chance for the oxygen gas to disperse outside of the battery. When charging is carried out with a large current in the sealed lead acid battery, a produced quantity of oxygen gas on the positive plate becomes larger than an absorbed quantity of it on the negative plate, so that hydrogen gas is also produced from the negative plate. For, this reason, when the battery is completely sealed, the inside of the battery is filled with oxygen gas and hydrogen gas so as to cause an increase in inside pressure. In the event when these gases catch fire by some accident, explosion of the battery would occur.
In order to restrain or prevent such an explosion of the battery, the following methods (a) and (b), for example, are tried now. (a) Method in which a safety valve that opens when the battery inside pressure rises is installed and the produced gas is exhausted by this valve. (b) Method in which a porous material is filled in a space on upper parts of positive and negative plate groups under a cover on a container.
In the method (a), however, an extent of explosion can be lessened but breaking-into-fragments of the container or other parts by the explosion can not be prevented. A method, in which the cover is put in place after filling plastic etc. in the space on upper parts of positive and negative plate groups, is generally adopted for the method (b). However, this method includes such problems that the work becomes troublesome and manufacturing cost increases.
On the other hand, when the sealed lead acid battery is overcharged during use, the positive plate will be corroded. Many of corroded materials produced in this instance are formed on grain boundaries of lead alloy composing the positive plate. Since the corroded material increases in its volume, it will widen the grain boundary so as to elongate the positive plate. FIG. 5 is a vertical sectional view showing a case where the positive plate is elongated. In FIG. 5, 10 denotes positive and negative plate groups composed of plural positive plates 1 and negative plates 2 arranged through separators 3. A lug 2a which is formed on an upper edge of the negative plate 2 and extends upward, is joined to a strap 4 from under side. When the positive plate 1 is corroded so as to be elongated, an elongated portion 11 will sometimes contact with the lug 2a or the strap 4 as shown by FIG. 5. For this reason, a short-circuiting has sometimes occurred so as to exhaust the life of the battery before the life of plate has been exhausted.
As a method for obstructing the elongation of the positive plate 1 to prevent the short-circuiting, a method is tried, for example, in which a combed plate made of plastic having acid resistance and insulation property is inserted in such a way that each combed piece is located between adjoining lugs 2a. However, this method was able to prevent the positive plate 1 from directly contacting with the strap 4 but was not completely able to prevent the positive plate 1 from contacting with the lug 2a. Therefore, the short-circuiting could not be prevented completely. The foregoing insertion of plate 1 was carried out by hand work after putting the positive and negative plate groups 10 in a container, but spaces between the lugs 2a could not be kept constant when welding the lugs 2a by means of a cast-on method used frequently in recent years. For this reason, the above plate-insertion work was difficult and automization of the insertion work was also difficult.
This invention is made in consideration of the problems described above. Namely, an object of this invention is to provide a sealed lead acid battery which can prevent the explosion of the sealed lead acid battery, prevent short-circuiting and provide a simple method of manufacturing a sealed lead acid battery.